Direct Monitoring of Li₂S₂ Evolution and Its Influence on the Reversible Capacities of Lithium-Sulfur Batteries

The polysulfide (PS) dissolution and low conductivity of lithium sulfides (Li₂S) are generally considered the main reasons for limiting the reversible capacity of the lithium-sulfur (Li-S) system. However, as the inevitable intermediate between PSs and Li₂S, lithium disulfide (Li₂S₂) evolutions are always overlooked. Herein, Li₂S₂ evolutions are monitored from the operando measurements on the pouch cell level. Results indicate that Li₂S₂ undergoes slow electrochemical reduction and chemical disproportionation simultaneously during the discharging process, leading to further PS dissolution and Li₂S generation without capacity contribution. Compared with the fully oxidized Li₂S, Li₂S₂ still residues at the end of the charging state. Therefore, instead of the considered Li₂S and PSs, slow electrochemical conversions and side chemical reactions of Li₂S₂ are the determining factors in limiting the sulfur utilization, corresponding to the poor reversible capacity of Li-S batteries.